1. Field of the Invention
This invention relates to an X-ray reflecting device in which a reflecting mirror is produced by a spherical surface machining method, and in which when the device is used, a desired aspherical reflecting surface can be obtained by elastically deforming the mirror with application of an external force.
2. Description of the Prior Art
A reflecting mirror with a spherical surface is easy to produce, but when used for condensing light, the spherical mirror cannot focus the light on a spot and causes spherical aberration. In order to eliminate the spherical aberration, it is preferable to use a reflecting mirror with an aspherical reflecting surface such as parabolic surface. However, it is difficult to finish the reflecting surface into a smooth aspherical shape.
In a finishing method used at present to obtain the aspherical surface, cutter movement is numerically controlled. However, in this method, the cutter movement is controlled in a stepping manner so that only an approximate aspherical surface is obtained, even if the cutter-feeding resolution is minimized. There is another machining method in which the aspherical surface is formed by cutting or grinding a material with an aspherical surface generator. However, in the last step of this method, polishing is required to finish an aspherical surface in the same manner as that in the numerical controlling. Another method has been known in which a material is machined with a spherical surface lathe, a spherical surface generator, or a spherical surface lap, and the machined spherical surface is polished into an aspherical surface.
Performance of such a reflecting device is evaluated by its reflectance and shape accuracy. Normally, allowable shape accuracy varies with light wavelengths, and requires about one-tenth the light wavelength .lambda.. In a case of, for example, soft X-ray, wavelength thereof ranges from about 5 to 50 nm. Therefore, the shape accuracy required to the reflecting mirror is 1 nm or less.
It is difficult to obtain such a high shape accuracy as required to an X-ray reflecting mirror by any of above described conventional machining methods. Thus, an X-ray aspherical reflecting mirror has not been put to practical use. It may be possible to gradually enhance the shape accuracy by repeatedly performing a shape modifying step. However, for most applications, this method is not practical since much time and high cost are required to manufacture the device.